1996 Chevy S-10 Stupid Question

I've done a little work on my truck, but I am by far a good mechanic. I've wanted to fix this for a while, but I really don't know where to start trouble shooting. If you could point me in the right direction, I'd be eternally grateful.

Problem:
After my air conditioner compressor went out on my truck years ago, my heater has been stuck on the feet and defrost position. No matter what I turn the dial to, it will only come out that one position. The heat still works fine, and it doesn't bother me too much. It's just annoying that it doesn't work.

Please help!
Thanks,
Steven